In a magnetic disk device, a voice coil motor has been conventionally used to position a magnetic head at a predetermined track. Recently, with the progress of a small and high-performance computer, there has been an increasing demand for a smaller magnetic disk device of a larger capacity and higher access. To satisfy the demand, the technology of high density track, and high-speed and high precision positioning is required, and requirements of the voice coil motor have become higher.
FIG. 25 shows a conventional voice coil motor.
The conventional voice coil motor includes a pair of yokes 1 arranged opposite to each other, a permanent magnet 2 fixed to at least one of the yokes 1, a fan-shaped movable coil 3 formed in a magnetic space defined between the yoke 1 and the permanent magnet 2 and being rotatable around a bearing 5, and a head carriage 4 coupled to the movable coil 3.
When a predetermined driving current flows through the fan-shaped movable coil 3, driving force is generated in the movable coil 3 according to the Fleming's left-hand rule, and the movable coil 3 rotates along an arrow A around the bearing 5, thereby rotating the head carriage 4 around the bearing 5 along an arrow B. As a result, a magnetic head 6 provided at the end of the head carriage 4 is moved opposite the rotation of the movable coil, thus being positioned.
However, the conventional voice coil motor has the problem that the positioning precision is lowered by the vibration of the head carriage 4 when the magnetic head is positioned by high-speed drive, thereby interfering with the high-density recording process on the magnetic disk device.
This problem will be described below in detail by referring to FIG. 26.
When an electric current flows through the fan-shaped movable coil 3, the driving force F is generated in the radial sections of the movable coil 3 by the magnetic field of the magnetic circuit formed by the permanent magnet 2 and the yoke 1. The resultant force generates a torque T around the bearing 5, and the electric current of the movable coil 3 can be controlled, thereby positioning the magnetic head 6.
At this time, a radial load of about 2F is applied to the bearing 5 with the torque T. When the flow of the current becomes opposite quickly in the movable coil 3 for high-speed drive, the direction of the radial load applied to the bearing 5 changes correspondingly, and therefore, the head carriage 4 vibrates along the arrow D.
Since the direction of the radial load applied to the bearing 5 is almost parallel to the direction C of the movement when the magnetic head 6 is positioned, the vibration of the head carriage 4 generated by the radial load deteriorates the positioning precision of the magnetic head 6. This lowers the tracking precision in a shorter track, thereby interfering with the improvement in recording density.
In a magnetic disk device, it is necessary to quickly position the magnetic head 6 with high precision. However, when the conventional voice coil motor is used, the vibration of the head carriage 4 lowers the positioning precision when the magnetic head 6 is positioned in high-speed drive. This interferes with a higher-density record on the magnetic disk device.
This phenomenon will be described below in detail by referring to FIG. 25. When an electric current flows through the movable coil 3, the magnetic field of the magnetic circuit formed by the permanent magnet 2 and the yoke 1 generates the driving force F in the radial sections of the movable coil 3. The resultant force generates the torque T around the bearing 5, and the electric current in the movable coil 3 is appropriately controlled, thereby positioning the magnetic head 6. At this time, the radial load of about 2F is applied to the bearing 5 with the torque T. When the direction of the flow of the electric current through the movable coil 3 is quickly switched for high-speed drive, the direction of the radial load applied to the bearing 5 changes correspondingly. As a result, the head carriage 4 vibrates along the arrow D. Since the direction of the radial load 2F applied to the bearing 5 is almost parallel to the movement direction C when the magnetic head 6 is positioned, the vibration of the head carriage 4 generated by the radial load 2F lowers the positioning precision of the magnetic head 6.
The present invention has been developed to solve the above-described problems, and aims at providing a swing-type voice coil motor capable of quickly positioning a rotating member with high precision without applying a radial load to a unit such as a bearing supporting a freely rotating member or the like when moving a rotating member such as a head carriage supporting a magnetic head or the like.